Fix Linkpoint gateway to sum the discount and subtotal

* The Linkpoint gateway requires that the subtotal, tax and shipping
  equal the payment total. It completely ignores the discount
  provided, which could be the result of a promo code.

  This change adds the discount to the subtotal, and sets the
  discount to 0. While not strictly necessary to set to 0, I did not
  want to remove it completely since it breaks the contract already
  established that the gateway options include a discount key.

Fixes #103